A conventional mobile communication system allows a mobile station to continue communication while moving by performing handover control between the mobile station and a radio base station.
To be more precise, when a mobile station in communication is about to move beyond the service area (cell) of a radio base station, the mobile station reports its radio quality to the radio base station to which the mobile station is being connected.
The radio base station receives the radio quality and judges whether or not to cause the mobile station to switch to a cell in which the mobile station can communicate with better radio quality.
If judging that the switching of a cell is necessary as a result of the above judgment, the radio base station instructs the mobile station to switch the cell (a handover).
For example, in a mobile communication system being studied as the Evolved universal terrestrial radio access network (E-UTRAN), a handover is carried out in accordance with procedures as shown in FIG. 1 (see Non-patent Document 1).
As shown in FIG. 1, in step 1, a connection source radio base station (Source eNB) 200A transmits a measurement control signal (Measurement Control) including a parameter concerning a radio quality measurement to a mobile station 100.
In step 2, when measured radio quality satisfies a condition for reporting, the mobile station 100 transmits a measurement result report signal (Measurement Report) including the measurement result to the connection source radio base station 200A.
In step 3, the connection source radio base station 200A judges whether or not to execute a handover in consideration of the measurement result reported through the measurement result report signal, usage of radio resources, and the like.
In step 4, the connection source radio base station 200A transmits a handover request signal (Handover Request) to a connection target radio base station (Target eNB) 200B.
The handover request signal is configured to transmit information necessary for the connection target radio base station 200B to execute a handover (including an identifier of a connection target cell, information related to the mobile station, and bearer information, for example).
In step 5, the connection target radio base station 200B judges whether or not to accept the mobile station 100 that is going to perform handover.
When judging that the mobile station 100 is acceptable, the connection target radio base station 200B reserves necessary resources and a cell specific mobile station identifier (C-RNTI: cell specific radio network temporary identifier) that the mobile station 100 uses after switching the connected cell.
In step 6, the connection target radio base station 2003 transmits a handover request acknowledgment signal (Handover Request Acknowledge) to the connection source radio base station 200A.
Here, the handover request acknowledgment signal includes information elements to be transmitted from the connection target radio base station 200B to the mobile station 100, and these information elements are transparently treated by the connection source radio base station 200A.
Concrete examples of the information elements to be transmitted from the connection target radio base station 200B to the mobile station 100 via the connection source radio base station 200A include the cell identifier that the mobile station 100 uses after switching the connected cell, i.e., the C-RNTI, radio related parameters, part of broadcast information of a connection target cell, and the like.
In step 7, the connection source radio base station 200A generates a handover command signal (Handover Command) which includes the above-described information elements having received in step 6 and is to be transmitted from the connection target radio base station 200B to the mobile station 100, and transmits the handover command signal to the mobile station 100.
The mobile station 100 moves on to a handover operation based on the information elements included in the received handover command signal. In step S8, the mobile station 100 establishes downlink synchronization with the connection target radio base station 200B, and transmits an uplink synchronization establishment request signal (Synchronization) to the connection target radio base station 200B in order to establish uplink synchronization with the connection target radio base station 200B.
In step 9, in response to the uplink synchronization establishment request signal, the connection target radio base station 200B transmits a timing information signal including uplink timing information (Timing Advance) and uplink allocation information (UL Grant) to the mobile station 100.
In step 10, after establishing the uplink synchronization, the mobile station 100 transmits a handover completion signal (Handover Confirm) to the connection target radio base station 200B for notification of handover completion.
However, the above-described mobile communication system has a problem of a waste of radio resources and a reduction in radio transmission efficiency.
Non-patent Document 1: 3GPP 36.300 V8.0.0 Technical specification group radio access network; Evolved universal terrestrial radio access (E-UTRA) and Evolved universal terrestrial radio access network (E-UTRAN); Overall description; Stage 2, 10.1.2.1.1